In a fuel cell system capable of carrying out highly-efficient small-scale electric power generation, it is easy to construct a system for utilizing heat energy generated during electric power generation. Therefore, such fuel cell system has been developed as a distributed electric power generating system capable of realizing high energy use efficiency.
In the fuel cell system, when carrying out an electric power generating operation, a hydrogen-containing gas and an oxygen-containing gas are supplied to a fuel cell stack (hereinafter simply referred to as “fuel cell”) provided as a main body of an electric power generating portion of the fuel cell system. Then, in the fuel cell, a predetermined electrochemical reaction proceeds by using hydrogen contained in the supplied hydrogen-containing gas and oxygen contained in the supplied oxygen-containing gas. By the progress of the predetermined electrochemical reaction, chemical energies of the hydrogen and the oxygen are directly converted into electric energy in the fuel cell. With this, the fuel cell system outputs electric power to a load.
Here, a means for supplying the hydrogen-containing gas necessary during the electric power generating operation of the fuel cell system is not usually built as an infrastructure. Therefore, a conventional fuel cell system is typically provided with a reformer configured to generate the hydrogen-containing gas necessary during the electric power generating operation. In the reformer, the hydrogen-containing gas is generated from the water and the raw material, such as a city gas, containing an organic compound by the progress of a steam-reforming reaction on a reforming catalyst. In this case, the reforming catalyst of the reformer is heated by a heater to a temperature appropriate for the progress of the steam-reforming reaction. Typically used as the heater is a combustion burner. The reforming catalyst of the reformer is heated by combusting a mixture gas of the city gas and air supplied by a combustion fan. Moreover, typically, a fuel off gas unconsumed in the fuel cell is combusted by the heater during the electric power generation or the like. Although the reformer is heated by the combusted gas, the combustion gas having been used for heating the reformer is still high in temperature. Therefore, in order to recover the heat of the combustion gas, the combustion gas is typically cooled down by the water using an exhaust gas heat exchanger. The water recovers the heat in the fuel cell system and is stored in a tank or the like as hot water of about 60 to 70° C. As above, a flue gas having recovered the heat is released through an exhaust port to outside atmosphere.
The reformer heated by the heater efficiently generates the hydrogen-containing gas from the water and the raw material, such as the city gas, by the reforming reaction. The fuel cell system generates the electric power by utilizing the hydrogen-containing gas generated by the reformer and, for example, the air as the oxygen-containing gas. Used as the water in the reforming reaction in the reformer is steam generated by a water evaporator provided in the hydrogen generator. In the case of especially reducing the energy loss and improving the efficiency of reforming, the water evaporator is typically included in the reformer.
The hydrogen-containing gas generated by the reformer contains carbon monoxide. Since the carbon monoxide poisons the catalyst contained in the fuel cell, the fuel cell cannot carry out electric power generation appropriately. In order to reduce a carbon monoxide concentration in the hydrogen-containing gas generated by the reformer, a shift converter for carrying out a shift reaction and a carbon monoxide remover for carrying out a selective oxidation reaction are typically provided. The reformer, the shift converter, and the carbon monoxide remover are collectively called a hydrogen generator.
Here, proposed is a fuel cell system in which since the hydrogen generator (reformer) is high in temperature when the fuel cell system stops operating, the combustion fan is activated after combustion stop of the combustion burner to supply cool air to the reformer, thereby cooling down the reformer (see PTL 1 for example).